Midterm Review Session

Transcription

1 Midterm Review Session Programming Problems For more practice: Copyright 2009 by Pearson Education

2 Midterm Logistics Bring your UW Student ID card!! Will check your ID while you re leaving Bring your stuff with you while you exit (don t go back to seat) 60 minute exam -10 if you start early or try to write after time Any paper notes allowed, but NO ELECTRONICS Book OK, section handouts OK, past exams + solutions OK, etc. NO CALCULATORS! NO LAPTOPS! Aiming for a median of 80 Will be a curve if the exam is too hard, will not have a hurtful curve if exam is too easy Copyright 2009 by Pearson Education 2

3 About the Exam 100 pts, 20% of course grade Non-programming 61 pts Expressions (10), parameter mystery (12), if/else simulation (12), while loop simulation (12), assertions (15) Easy Programming 15 pts Medium Programming 15 pts Really Hard Programming 9 pts Topics covered: Chapters 1-5 Topics not covered: see website! Includes Graphics, printf, do/while, break Copyright 2009 by Pearson Education 3

4 More Exam Info Non-programming questions You do not need to show your work You are not allowed to use calculators (e.g. on expressions) Programming questions Are not graded on style, only external correctness Must write valid Java code that would compile Will not grade comments or pseudo-code Do NOT use abbreviations Do not have to write import statements Substantial partial credit is given for partially working solutions Write as much as you can: even a method header and the beginnings of a while loop could earn you partial points! More info on course website! Copyright 2009 by Pearson Education 4

5 General Tips BE AFRAID. Success on HW is not an indication of being prepared for the exam. Watch the clock! The hardest part about this exam will be the time limit. PRACTICE Q1-Q5 so that you can go through them fast and accurately! You want as much time as possible for the programming questions. Write as much as you can! A partial answer is better than none at all. Writing the correct method header will earn at least 1 point. Writing a partial loop, if statements, returns, etc. will points too Copyright 2009 by Pearson Education 5

6 General Tips Read the instructions now and learn the format so you won't be surprised tomorrow The format of the midterm will be the same as the samples For more practice, use Marty Stepp's Practice-it tool: Copyright 2009 by Pearson Education 6

7 Concepts Review Some essential topics from Ch1-5 Copyright 2009 by Pearson Education

8 For-loops Use to repeat stuff when you know exactly* how much you want to repeat * Could also have a variable or expression that tells you exactly how much you want to repeat Nested for-loops: loops within a loop When you want to do task multiple times, and that task has repetition to it E.g. multiplication table Cumulative sum: variable that keeps a sum in progress and is updated repeatedly until summing is finished In general: think about bounds, possible fencepost issues Copyright 2009 by Pearson Education 8

9 Conditionals and Tests If/else statement: Decide between several logical choices While loop: Repeat unknown number of times Sentinel loops: repeat until a certain signal is seen (e.g. prompt until you see a -1) boolean: primitive type with a true or false value When to return?? Boolean flag is helpful Copyright 2009 by Pearson Education 9

10 Strings length() to get length of a String indexof/contains to search for substring within String equals/equalsignorecase to test equality startswith/endswith to test beginning/end of word substring(index1, index2) to get a piece of a String index1 inclusive, index2 exclusive Index 0 for first letter, index length() -1 for last letter + to concatenate Strings together Copyright 2009 by Pearson Education 10

11 Other miscellaneous stuff Scanner to prompt for input hasnext, hasnextint, hasnextdouble, hasnextline Random to get a random value To generate random number between [min, max]: r.nextint(range) + min Where range: max min + 1 r.nextint(51) + 50; // produces random value Expressions stuff: % 10 and / 10 to get digits % 2 == 0 to test for even (!= for odd) Copyright 2009 by Pearson Education 11

12 Programming Practice Practice problems from previous exams Copyright 2009 by Pearson Education

13 Programming Tips Read the problems carefully: Does it want you to print a result, or return it? What values does the method use for computation? Are these values parameters, are they read from a Scanner, etc.? What type of value (if any) does the method return? Have you handled all special cases? What if the integer is 0, or negative? What if the string has no letters? What if there is only one word in the string? Many words? Copyright 2009 by Pearson Education 13

14 Practice Problem 1: Print Multiples Write a static method named printmultiples Takes two integers n and m as parameters and prints the first m multiples of n Assume m >= 1 Multiples are separated by commas printmultiples(3, 5); The first 5 multiples of 3 are 3, 6, 9, 12, 15 printmultiples(7, 3); The first 3 multiples of 7 are 7, 14, 21 Copyright 2009 by Pearson Education 14

15 Print Multiples Solution public static void printmultiples(int n, int times) { System.out.print("The first " + times + " multiples of " + n + " are " + n); for (int i = 2; i <= times; i++) { System.out.print(", " + i * n); System.out.println(); Copyright 2009 by Pearson Education 15

16 Practice Problem 2: Count Even Digits Write a static method named countevendigits Accepts an integer as its parameter and returns the number of even-valued digits in that number. Even digits: 0, 2, 4, 6, or 8 Assume value passed to your method is non-negative // 4 even digits: two 8s, the 4, and 6 int x = countevendigits( ) ; System.out.println("x is " + x); // x is 4 Copyright 2009 by Pearson Education 16

17 Count Even Digits Solution public static int countevendigits(int n) { int count = 0; while (n!= 0) { int digit = n % 10; n = n / 10; if (digit % 2 == 0) { count++; return count; Copyright 2009 by Pearson Education 17

18 Practice Problem 3: Cheerleader Write a static method named cheerleader Takes two params: number of lines of output number of "cheers" per line Cheer structure: 1 cheer: Go 2 cheers: Go Team Go 3 cheers: Go Team Go Team Go Each line indented by 3 spaces; first line at 0 spaces cheerleader(2, 1); Go Go cheerleader(4, 3); Go Team Go Team Go Go Team Go Team Go Go Team Go Team Go Go Team Go Team Go cheerleader(2, 4); Go Team Go Team Go Team Go Go Team Go Team Go Team Go Copyright 2009 by Pearson Education 18

19 Cheerleader Solution public static void cheerleader(int lines, int cheers) { for (int line = 1; line <= lines; line++) { for (int space = 1; space <= line - 1; space++) { System.out.print(" "); for (int cheer = 1; cheer <= cheers - 1; cheer++) { System.out.print("Go Team "); System.out.println("Go"); Copyright 2009 by Pearson Education 19

20 Practice Problem 4: Favorite Letter Write a static method named faveletter Accepts two parameters: Scanner for the console Favorite letter represented as a one-letter String Repeatedly prompt the user until two consecutive words are entered that start with that letter. Case sensitive (assume a Scanner named console was made earlier in code) faveletter(console, "y"); Looking for two "y" words. Type a word: hi Type a word: bye Type a word: yes Type a word: what? Type a word: yellow Type a word: yippee "y" is for "yippee" Print a message showing the last word typed. Copyright 2009 by Pearson Education 20

21 Favorite Letter Solution public static void faveletter(scanner console, String letter) { System.out.println("Looking for two \"" + letter + "\" words."); int count = 0; String word = ""; while (count < 2) { System.out.print("Type a word: "); word = console.next(); if (word.startswith(letter)) { count++; else { count = 0; System.out.println("\"" + letter + "\" is for \"" + word + "\""); Copyright 2009 by Pearson Education 21

22 Practice Problem 5: Random Rects Write a static method named randomrects Calculates and displays the area of randomly-generated rectangles. Each rectangle has random width and height btwn 1 and 10 inclusive Keep generating rectangles until an increasing sequence of four areas is printed. Stop when the last four rectangles generated have areas of a1, a2, a3 and a4 such that a1 < a2 < a3 < a4 randomrects(); w: 5, h: 6, area: 30 w: 10, h: 5, area: 50 w: 2, h: 8, area: 16 w: 4, h: 4, area: 16 w: 2, h: 9, area: 18 w: 8, h: 3, area: 24 w: 7, h: 2, area: 14 w: 3, h: 10, area: 30 w: 7, h: 9, area: 63 w: 9, h: 8, area: 72 End random rectangles. Copyright 2009 by Pearson Education 22

23 Random Rectangles Solution public static void randomrects() { Random r = new Random(); int last = 0; int count = 0; while (count!= 4) { int w = r.nextint(10) + 1; int h = r.nextint(10) + 1; int area = w * h; if (area <= last) { count = 1; // need to count first rect in sequence else { count++; System.out.println("w: " + w + ", h: " + h + ", area: " + area); last = area; System.out.println("End random rectangles."); Copyright 2009 by Pearson Education 23

24 Non-Programming Practice Tips on Questions 1-4 (For assertions help, see lecture 14) Copyright 2009 by Pearson Education

25 Expressions integer division and mod: quotient and remainder 14 / 3 is 4, 14 % 3 is 2 7 / 10 is 0, 7 % 10 is 7 precedence: ( ) before * / % before * 6 - (1 + 7) % * 6-8 % % Copyright 2009 by Pearson Education 25

26 Expressions 2 String concatenation: same precedence as integer + -, evaluated left-to-right with other + - operations "3" "3" "33" "334" + 5 "3345" type promotion: done as needed when int and double are mixed 50 / 6 / / Copyright 2009 by Pearson Education 26

27 Expression questions Evaluate the following expressions: 16 / * 2 8 / 7 + "5 / 4" + 8 / 3 88 % 10 % 3 * 16 / / 3 / 2 / * (3 + 2 * 6) / (8-14 / 3) * 2.2 Copyright 2009 by Pearson Education 27

28 Expression answers Correct answers: 16 / * / 7 + "5 / 4" + 8 / 3 "15 / 42" 88 % 10 % 3 * 16 / / 3 / 2 / * (3 + 2 * 6) / (8-14 / 3) * Copyright 2009 by Pearson Education 28

29 Parameter mystery question What is the output of the following program? public class Mystery { public static void main(string[] args) { String john = "skip"; String mary = "george"; String george = "mary"; String fun = "drive"; String work = "john"; speak(mary, john, fun); speak(george, work, john); speak(fun, "george", "work"); speak(george, mary, john); speak(george, "john", "dance"); public static void speak(string mary, String john, String fun) { System.out.println(john + " likes to " + fun + " with " + mary); Copyright 2009 by Pearson Education 29

30 Parameter mystery tips Try making a table of the parameters passed to each call: public class Mystery { public static void main(string[] args) { String john = "skip"; String mary = "george"; String george = "mary"; String fun = "drive"; String work = "john"; speak(mary, john, fun); speak(george, work, john); speak(fun, "george", "work"); speak(george, mary, john); speak(george, "john", "dance"); george skip drive mary john skip drive george work mary george skip mary john dance public static void speak(string mary, String john, String fun) { System.out.println(john + " likes to " + fun + " with " + mary); Copyright 2009 by Pearson Education 30

31 Parameter mystery answer What is the output of the following program? public class Mystery { public static void main(string[] args) { String john = "skip"; String mary = "george"; String george = "mary"; String fun = "drive"; String work = "john"; speak(mary, john, fun); speak(george, work, john); speak(fun, "george", "work"); speak(george, mary, john); speak(george, "john", "dance"); skip likes to drive with george john likes to skip with mary george likes to work with drive george likes to skip with mary john likes to dance with mary public static void speak(string mary, String john, String fun) { System.out.println(john + " likes to " + fun + " with " + mary); Copyright 2009 by Pearson Education 31

32 While loop mystery question Given the following program, public static void mystery(int y) { int x = 0; int z = 0; while (y > 0) { System.out.print(x + " " + z + " "); x++; z = z + y % 10; y = y / 10; System.out.println(x + " " + z); What is the output of the following sequence of calls? mystery(0); mystery(8); mystery(32); mystery(72); mystery(184); mystery(8239); Copyright 2009 by Pearson Education 32

33 While loop mystery tips Keep track of each variable's value as it changes: public static void mystery(int y) { int x = 0; int z = 0; while (y > 0) { System.out.print(x + " " + z + " "); x++; z = z + y % 10; y = y / 10; System.out.println(x + " " + z); What is the output of the following call? mystery(184); Sometimes, these problems are performing real computations in disguise. What is this problem really doing? x y z Copyright 2009 by Pearson Education 33

34 While loop mystery answers Given the following program, public static void mystery(int y) { int x = 0; int z = 0; while (y > 0) { System.out.print(x + " " + z + " "); x++; z = z + y % 10; y = y / 10; System.out.println(x + " " + z); What is the output of the following sequence of calls? mystery(0); 0 0 mystery(8); mystery(32); mystery(72); mystery(184); mystery(8239); Copyright 2009 by Pearson Education 34